Electronic circuits are today used in almost all products, and in particular in products related to transfer of information. Such transfer of information can be done along wires and cables at low frequencies (e.g. wire-bound telephony), or wireless through air at higher frequencies using radio waves both for reception of e.g. broadcasted audio and TV, and for two-way communication such as in mobile telephony. In the latter high frequency cases both high and low frequency transmission lines and circuits are used to realize the needed hardware. The high frequency components are used to transmit and receive the radio waves, whereas the low frequency circuits are used for modulating the sound or video information on the radio waves, and for the corresponding demodulation. Thus, both low and high frequency circuits are needed. The present invention relates to a new technology for realizing high frequency components such as transmitter circuits, receiver circuits, filters, matching networks, power dividers and combiners, couplers, antennas and so on.
The first radio transmissions took place at rather low frequency below 100 MHz, whereas nowadays the radio spectrum (also called electromagnetic spectrum) is used commercially up to 40 GHz, and some systems for higher frequencies are planned and even to some degree in use already. The reason for the interest in exploring higher frequencies is the large bandwidths available. When wireless communication is spread to more and more users and made available for more and more services, new frequency bands must be allocated to give room for all the traffic. The main requirement is for data communication, i.e. transfer of large amounts of data in as short time as possible.
There exist already transmission lines for light waves in the form of optical fibers that can be buried down and represents an alternative to radio waves when large bandwidth is needed. However, such optical fibers also require electronic circuits connected at either end. There may even be needed electronic circuits for bandwidths above 40 GHz to enable use of the enormous available bandwidths of the optical transmission lines. The present invention can be used to realize electronic circuits above typically 40 GHz where there exist no good alternatives solution today for low loss and mass production.
Electronic circuits below typically 300 MHz (i.e. wavelengths longer than 1 meter) are easily realized in printed circuit boards (PCB) and in integrated circuits using designs based on concentrated circuit elements such as resistors, inductors, capacitors and transistor amplifiers. Such technology may also work at higher frequency, but the performance degrades gradually when the size of the PCB and integrated circuit package become comparable to a wavelength. When this happens, it is better to realize the circuits by connecting together in various ways pieces of transmission lines or waveguides. This is normally referred to as microwave technology and is commonly in use between 300 MHz and 30 GHz, i.e. the microwave region. The most common transmission lines are coaxial cables and lines, microstrip lines, and cylindrical waveguides. There are problems with these technologies for higher frequencies than 30 GHz because of increasing losses and manufacturing problems (smaller dimensions and stricter tolerance requirements). The tolerance requirements could be some pro mille ( 1/1000) of a wavelength, which becomes very small when recalling that the wavelength is 10 mm at 30 GHz. Also, the coaxial lines and waveguides need to be thinner than typically 0.5 wavelengths to work with a required single mode. Such hollow lines and guides are very difficult to manufacture, which makes it necessary at high frequency to instead use microstrip lines and other substrate-bound transmission lines. However, substrate-bound transmission lines have larger losses that increase with increasing frequency, so the performance degrades. The output power of transistors is lower at such high frequencies, and when they are mounted into lossy transmission lines the power generation becomes even a larger problem. The present invention relates to electronic circuits made by using a new transmission line that at high frequencies is advantageous with respect to losses and manufacturability.
There exist already some waveguides particularly intended for use at high frequencies because they have lower losses and are cheaper to manufacture than traditional air-filled cylindrical waveguides and because they have lower losses than microstrip lines. Such a waveguide is the so-called Substrate Integrated Waveguide (SIW), as described in J. Hirokawa and M. Ando, “Single-layer feed waveguide consisting of posts for plane TEM wave excitation in parallel plates,” IEEE Trans. Antennas Propag., vol. 46, no. 5, pp. 625-630, May 1998. Here, the waveguide is made in the substrate of a PCB by using metalized via holes as walls. These waveguides still suffer from losses due to the substrate, and the metalized via holes represent a complication that is expensive to manufacture. The present invention does not necessarily make use of via holes and substrate to provide a high frequency waveguide, but it can make use of such if needed of other reasons.
The last 8-10 years researchers all over the world have tried to synthesize artificial electromagnetic materials that have abnormal characteristics. Such materials are often referred to as metamaterials, and one of the most desirable abnormal characteristics to achieve in electronics is the equivalent of magnetic conductivity, which does not exist in nature. The first conceptual attempt to realize magnetic conductivity described in the scientific literature was the so-called soft and hard surfaces, see P-S. Kildal, “Artificially soft and hard surfaces in electromagnetics”, IEEE Trans. Antennas Propagat., Vol. 38, No. 10, pp. 1537-1544, October 1990. The ideal soft and hard surfaces are nowadays most conveniently described as PEC/PMC strip grids, i.e. grids of parallel strips, where every second strip is perfectly electric conducting (PEC) and perfectly magnetic conducting (PMC), respectively, see P.-S. Kildal and A. Kishk, “EM Modelling of surfaces with STOP or GO characteristics—artificial magnetic conductors and soft and hard surfaces”, Applied Computational Electromagnetics Society Journal, Vol. 18, No. 1, pp. 32-40, March 2003. The PMC strips are realized by metal grooves with effectively quarter wavelengths depth, or by equivalent means such as metal strips on a grounded substrate with metallised via holes between the strips and the via holes. The characteristics of the PEC/PMC strip grids are that the anisotropic boundary conditions allow waves of arbitrary polarization to propagate along the strips (hard surface case), whereas they stop wave propagation in other directions along the surface and in particular orthogonally to the strips (soft surface case). Such PEC/PMC strip grids can be used to realize new antenna types, see P.-S. Kildal, “Strip-loaded dielectric substrates for improvements of antennas”, U.S. patent application Ser. No. 10/495,330—Filed Nov. 12, 2002. The present invention makes use of the soft and hard surfaces and PEC/PMC strip grids to realize a high frequency waveguide that was not foreseen in U.S. patent application Ser. No. 10/495,330.
The so-called electromagnetic bandgap (EBG) surface stops wave propagation in a similar way as the soft surface, but for all directions of propagation. This appeared for the first time in the scientific literature in the following paper by D. Sievenpiper, L. J. Zhang, R. F. J Broas, N. G. Alexopolous, and E. Yablonovitch, “High-impedance electromagnetic surfaces with a forbidden frequency band”, IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 11, pp. 2059-2074, November 1999. Both Kildal's soft surface and Sievenpiper's EBG surface stop wave propagation along the surfaces, and they contain the PMC as an important surface component. Sievenpiper's invention has resulted in a number of patents, but the present invention is not described in them.
The propagation characteristics along soft and hard surfaces are quite well known, both when they are used in waveguides and as open surfaces, see e.g. S. P. Skobelev and P.-S. Kildal, “Mode-matching modeling of a hard conical quasi-TEM horn realized by an EBG structure with strips and vias”, IEEE Transactions on Antennas and Propagation, vol. 53, no. 1, pp. 139-143, January 2005, and Z. Sipus, H. Merkel and P-S. Kildal, “Green's functions for planar soft and hard surfaces derived by asymptotic boundary conditions”, IEE Proceedings Part H, Vol. 144, No. 5, pp. 321-328, October, 1997. However, the studies have been limited to cylindrical waveguides and open surfaces, respectively. The present invention creates instead local transmission lines, waveguides and circuit components between parallel conductors and makes use of special techniques to prevent spreading of the waves between the conductors and to suppress undesired higher order modes.
There has been other attempts to make high frequency metamaterial waveguides, such as in George V. Eleftheriades, Keith G. Balmain, “Metamaterials for controlling and guiding electromagnetic radiation”, U.S. Pat. No. 6,859,114—Filed Jun. 2, 2003. However, this and other related solutions make use of wave propagation inside the metamaterial, or at the surface of it, both of which cause losses and large dispersion. Dispersion means that the bandwidth becomes narrow. The present invention controls wave propagation between parallel conducting plates, and it has lower losses and potentially a much larger bandwidth than U.S. Pat. No. 6,859,114.